Infrared transmitting glasses



Jan. 28, 1964 G. w. CLEEK ETAL 3,119,703

INFRARED TRANSMITTING GLASSES Filed July 31, 1961 4 Sheets-Sheet 1 LOmmWAVELENGTH, MIGRONS FIG. I

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INVENTORS 6/ V5 N K. 0L EEK E 064/? H. HAMILTON .6 BY mArToRNe-Ys UnitedStates Patent 3,119,703 INFRARED TRANSMHTKNG GLA E Given W. Qleeir,Arlington, Va, and Edgar H. Hamilton,

Shiver Spring, Md assignors to the United States of America asrepresented by the Secretary of the Navy Filed July 31, 1961, Ser. No.128,641 5 Claims. (Cl. 16--47) (Granted under Title 35, US. (lode(1952), see. 266) The invention described herein may be manufactured andused by or for the Government of the United States of America forgovernmental purposes without the payment of any royalties thereon ortherefor.

This invention relates to glass compositions and more particularly toinfrared transmitting glass compositions based on the BaOTiO -GeOternarysystem.

This application is a continuation-in-part of application Serial No.598,482, filed July 17, 1956, now US. Patent 3,022,182.

Application of infrared transmitting glasses in the field of optics hasbecome more prevalent and diversified due largely to thedemands'presented by military usages, such as in optical lenses foraerial photography equipment and fire control instruments operating inwavelength ranges from the visible region to 5.0 microns. Stringentrequirements of physical and chemical properties for infraredtransmitting glasses have been made necessary due to the wide range ofenvironmental factors surrounding practical applications of the deviceswhich employ these glasses.

Research directed toward the development of serviceable glasses based onternary systems, such as K O-CaOSiO and K OPbOSiO has led to thediscovery of commercial glasses which are fluid enough at anindustrially accessible temperature to be meltedon a commercial scale,viscous enough to be worked above its freezing point so thatdevitrification cannot take place, and which have physical propertiesand chemical durability suitable for the purpose for which they areintended. However, studies made to lead to the production ofspecial-type glasses capable of resisting chemical and physical changeordinarily brought about through exposure to extremes of heat and coldand corrosive chemicals have resulted in the finding of relatively fewacceptable glasses, particularly those needed in the infraredtransmitting ranges principally due to the strong water absorption bandfound in most alkali-containing glasses at about 2.75 microns.

An object of the present invention is to provide infrared glasses ofhigh refractive index.

Another object of the invention is the provisions of infraredtransmitting glasses having high deformation temperatures.

Another object is to provide glasses having good chemical durabilitiessuch as resistance-to attack over the entire pH range and lowhygroscopicity.

A further object is to provide infrared glasses of a composition whichenables the glass to be cooled from a molten state withoutcrystallization.

A still further object of the invention is the provision of a glasshaving good transmittances in the near infrared at wave lengths belowsix microns.

Another object is to provide multi-component glass compositions based onthe BaO-TiO GeO ternary system which may be formed in relatively largeamount melts.

Another object is to make alkali-free glasses having relatively lowabsorption in the 2.75 to 3.00 micron region.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawings wherein:

FIG. 1 is a graph showing transmittance curves for various thicknessesof a specific glass of this invention.

FIGS. 2 and 3 are graphs showing transmittance'curves and absorptioncurves for glasses of this invention.

PEG. 4 is a graph showing an expansion curve for a specific glass ofthis invention.

According to the present invention it has been found that multicomponentglasses having desirable characteristic properties may be developed fromthe ternary base glasses of the BaOTiO GeO system. Not only has it beendiscovered that glasses comprising the present invention are feasible intheir intended use in optical lens devices requiring infrared wavetransmission, but also that because of their relatively low meltingtemperature of from 13001400 C. the glasses may be prepared in largemelts with less fear of contamination, a factor which contributes tomass production of the glasses in distinction -to laboratory productionof limited amounts.

Examples of specific compositions which are preferred embodiments of thepresent invention are as follows:

TABLE I Compositions of Ge0 Glasses [Mole Percent] Glass NumberComponent F234 F235 F997 F998 10 12 Liquidus Temperature, o 1, 233 1,220 i i623 1,105 Max. Melting Temperature,

C 1, 410 1, 35 1, 340 1, 345 ND 1 97552 1. 99284 1. 86410 1. 85477 (Itis important to note that these glasses have refrac tive indices, Nabove 1.85 and one of them, F235, has a value of 1.99.)

These preferred glasses of the present invention containing BaO, TiO andGeO as the principal constituents have been found to have highrefractive indices, good transmittances in the near infrared at wavelengths below six microns, high deformation temperaturesydesirableelastic constants, and good chemical durabilities.

The refractive indices of these glasses are comparable to extra denseflints as can be seen in the following table.

3 TABLE 11 Refractive Indices of GeO Glasses Glass Number Wavelength,microns It is -well known in the art of glass making that extra denseflint glasses have PbO as a major constituent, and,

consequently, are not chemically durable. For example, one type ofantirefiection coating applied in alkaline solution cannot be used onglasses with high PbO contents due to their poor resistance to attack byalkaline solutions. The instant glasses, however, have low PbO contents,yet exhibit refractive indices comparable to that of PbO containingglasses.

The good transmittances of these glasses in the near infrared at wavelengths below six microns are evident from FIGS. 1, 2, and 3 which showthe properties of several glasses of the present invention for variousthicknesses of the glasses. Consequently, the present invention providesinfrared glasses with high refractive indices while at the same timeprovides good transmittances. This is significant in that known highrefractive index glasses usually contain appreciable amounts of boronoxide, but which glasses have little or no transmittances beyond wavelengths of 2.9 microns.

The deformation temperatures, as determined by the thermal expansionmethod, of exemplary glasses of the invention range from 700783 C. Forpurposes of illustration, the thermal expansion curve for glass F234 isgiven in FIG. 4. It is important to note that the deformationtemperature of 783 C. is well above that of extra dense flint glasses ofcomparable refractive indices, and is well above the deformationtemperatures of most commercial types of glass. For the other glasses itwas found that: F235 had a deformation point of 763 C., a criticaltemperature of 749 C., and a coefficient of thermal expansion of 8.8 10"in the IOU-700 C. range; F997 had a deformation temperature of 725 C., acritical temperature of 685 C., and a coefiicient of thermal expansionof 9.3 l0- in the 100-600 C. range; and F998 had a deformationtemperature of 700 C., a critical temperature of 665 C., and acoefficient of thermal expansion of 10 1O- in the IOU-600 C. range.

The elastic constants of the glasses of this invention are high comparedto the standard types of glasses. For example the elastic constants ofglass F998 are as follows:

TABLE III Elastic Constants of Glass F998 These indices of elasticconstants depict the physical flexibility of these glasses to the extentthat they are capable of withstanding physical punishment to a muchgreater degree than is normal for glasses. An attribute most desirablefor glasses of military application where in the field they are oftensubjected to rigorous physical requirements.

The chemical durabilities of the glasses were determined by aninterferometric method. A polished sample of each glass was immersedabout one-half its length in a solution buffered to the desired pH.After six hours exposure at C. the samples were viewed through anoptical flat with the aid of monochromatic light. Inasmuch as any shiftsin the interference bands as they passed from the unexposed to theexposed portion of the sample were proportional to the amount of attackby the buffering media, a measure of chemical durability of the samplewas accurately determined. The results of tests on the samples show nodetectable attack over the entire pH 2 to pH 12. The resistance of theseglasses to chemical attack over the entire pH range and their lowhygroscopicity make them unique as compared to commercial types ofglass.

It has been revealed by this invention that small amounts of otheroxides, such as ThO Ta O W0 PbO, CaO, SrO, SnO, CdO, and A1 0 also maybe included as glass components. The use of BaF replacing some of theBaO in the batch composition, has a beneficial effect on the removal ofthe water absorption band at 2.75 to 3.00 microns as seen by comparisonof the transmittance curves of glasses F997 and P998 in FIGS. 2 and 3.

The glasses of the instant invention are useful where infraredtransmitting glasses are required as in fire control and other opticaldevices, for making achromatically corrected lens systems which needglasses having high refractive indices and dispersions over aconsiderable range, and where good chemical durability is necessary forwithstanding chemical attack as in antirefiection coatings applied tolens elements.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

1. A non-crystalline infrared transmitting glass having a highrefractive index, good transmittances at wavelengths below six microns,high deformation temperatures, and good resistance to chemical attackover the entire pH range consisting of:

3545 mole percent of GeO 18-22 mole percent of BaO 10-30 mole percent ofTiO approx. 7 mole percent of La O 3-5 mole percent of ZrO and at leastone member of the group consisting of approximately 3 mole percent ofThO 1 mole percent of Ta O 2 mole percent of PhD, 2 mole percent of W010 mole percent of ZnO, and 4 mole percent of BaF;.

2. A non-crystalline infrared transmitting glass having a highrefractive index, good transmittances at wavelengths below six microns,high deformation temperatures, and good resistance to chemical attackover the entire pH range consisting of:

36 mole percent of Ge0 20 mole percent of BaO 30 mole percent of TiO 7mole percent of La o 3 mole percent of ZrO 3 mole percent of ThO 1 molepercent of Ta O 3. A non-crystalline infrared transmitting glass havinga high refractive index, good transmittances at wavelengths below sixmicrons, high deformation temperatures, and good resistance to chemicalattack over the entire pH range consisting of:

35 mole percent of Ge 18 mole percent of B20 30 mole percent of TiO 7mole percent of La O 3 mole percent of ZrO 3 mole percent of T110 1 molepercent of Ta O 2 mole percent of PbO 2 mole percent of W0 45 molepercent of GeO 22 mole percent of BaO 10 mole percent of TiO 7 molepercent of La O 5 mole percent of ZrO 1 mole percent of Ta O 10 molepercent of ZnO.

5. A non-crystalline infrared transmitting glass having a highrefractive index, good transmittances at wavelengths below six microns,high deformation temperatures, and good resistance to chemical attackover the entire pH nange consisting of:

mole percent of GeO 18 mole percent of B210 10 mole percent of TiO 7mole percent of La O 5 mole percent of ZrO 1 mole percent of Ta O 10mole percent of ZnO 4 mole percent of BaF References Cited in the fileof this patent UNITED STATES PATENTS 2,691,599 Blau Oct. 12, 19542,701,208 Blau Feb. 1, 1955 3,022,182 Cleek et a1. Feb. 20, 19623,029,152 Milne Apr. 10, 1962

1. A NON-CRYSTALLINE INFRARED TRANSMITTING GLASS HAVING A HIGHREFRACTIVE INDEX, GOOD TRANSMITTANCES AT WAVELENGTHS BELOW SIX MICRONS,HIGH DEFORMATION TEMPERATURES, AND GOOD RESISTANCE TOCHEMICAL ATTACKOVER THE ENTIRE PH RANGE CONSISTING OF: